Studies on the scars of humpback dolphins, Sousa chinensis, in Taiwan
Date Issued
2012
Date
2012
Author(s)
Lin, Ming-Ching
Abstract
The humpback dolphins (Sousa chinensis) off the west coast of Taiwan are listed as a critically endangered species in the IUCN list that call for instant protection. Scars on the dolphins can be studied as records of the environmental or predatory threats these cetaceans encountered. In this study, I used photos obtained from boat surveys and necropsy reports of stranded/ by-caught humpback dolphins to examine the scars on them and to elucidate the anthropogenic and natural threats they may have encountered. There scars are classified into six types of anthropogenic scars and one type of natural predatory scars caused by shark bites. Of the 71examined Sousa, 42% carried anthropogenic scars, and the annual increase rate of anthropogenic scars was 2±0.707 (scars/year) in this population. The scarring ratio increased with age, and it increased sharply from the speckled stage to spotted adults. Although the stranding rate was very low in this population, the scarring rate on the other hand was relatively higher than those of the Sousa populations in Hong Kong and South Africa. The dolphins bearing impression marks and propeller wounds, the two types of scars most likely related to anthropogenic activities, were most frequently sighted in coastal waters from Miaoli to Taichung and between Taichung and north Changhua, overlapping with the areas characteristic of heavy vessel traffic and fishery activities.
With regard to natural predatory scars, all the scars we examined were determined to be shark bites. Of the 71 dolphins 25.4% displayed shark bites. The annual increase rate of shark bites was 0.8±0.98, which was similar to the result found in Natal, South Africa. The consistency might be related to the similar salinity of the dolphins’ and sharks’ habitats. The predatory scarring ratio also increased fast during the period bwtween the speckled-stage and spotted adults. Among the sexually mature, scarred dolphins, 43.75% were females. This result may be associated with the adult dolphins’ protecting behaviors of the infants and calves. Based on the fish fauna records in Taiwan, the predatory scars are probably inflicted by the oceanic whitetip sharks (Carcharhinuslongimanus) and tiger sharks (Galeocerdocuvieri).
This study reveals that humpback dolphins in Taiwan face both specific anthropogenic and environmental threats. Although it is difficult to reduce the natural predatory pressure on the dolphins of this vulnerable population, reducing the anthropogenic threats must be a priority task for their conservation.
Subjects
Sousa chinensis
scars
anthropogenic influence
predatory pressures
SDGs
Type
thesis
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